The field of application of the invention extends to process control systems in which organic compounds such as hydrocarbons in gases are measured and monitored, for example in the production of gases or the measurement of emissions from combustion of fossil fuels. In the commonly known prior art, flame ionization detectors (FIDs) are provided for this purpose, in which carbon-containing molecules in a gas flame are ionized and detected by measuring a current flow.
In order to generate a flame that is hot enough to ionize molecules, a hydrogen flame is needed. So-called combustion air provides the necessary oxygen. The hydrocarbons are admixed through the gaseous analyte to be measured and conveyed by a burner nozzle into a combustion chamber. The hydrogen combusts with the oxygen above the burner tip of the burner nozzle to form water. The hydrocarbons contained therein also combust and form carbon dioxide as well as water.
Inside the flame there is a zone having temperatures of from 1000 to 1500° C., in which ionized intermediates such as CHO+ ions are produced. These ions are drawn out of the flame by an applied electric field before they can completely convert into CO2 and H2O. The positively charged CHO+ ions are carried away via a cathode, thereby generating an electric current that is tapped via an amplifier.
In order to produce a CHO+ ion, an oxygen and a hydrogen atom must be present in the flame, which bond to a carbon atom. If the concentration of oxygen in the gaseous analyte increases, there will be more oxygen atoms available for the reaction, whereby the conversion rate of C—H bonds increases, whereas the amount of hydrocarbons remains the same. This oxygen cross-sensitivity is a well-known disadvantage of FIDs.
In order to reduce the problem of oxygen cross-sensitivity, the combustion air is passed into the combustion chamber separately from and before the combustible gas, instead of mixing said air with the combustible gas and gaseous analyte on the input side of the burner nozzle. The oxygen must then diffuse from the outside into the flame (diffusion flame).
Apart from cross-sensitivity, another fundamental problem is the substance-dependent sensitivity of the FID. The number of C—H bonds per molecule of the gaseous analyte increases the measured amperage. For example, methane, having four H-bonds per carbon atom, produces a higher probability of ionization than propane, which has a maximum of three H-bonds per carbon atom. The substance-dependent transformation ratio is one of the most important metrological characteristics of FIDs and is called a response factor.
DE 203 20 366 U1 discloses a FID in which a burner nozzle having two or more holes is used. This produces a plurality of flames that may, under the right circumstances, merge to form a large flame having an increased surface area. This allows more oxygen to diffuse into the flame, which is intended to reduce the oxygen error.
Fundamentally, the development and optimization of a FID is costly due to the multitude of influencing factors. Thus, although a FID as described here can be optimized to a very low oxygen error, this usually leads to other metrological characteristics deteriorating at the same time. The reason for this is that a low oxygen error in FIDs having a diffusion flame is achieved at low hydrogen and combustion air amounts or low combustion air pressure. This causes the flame to drop and widen, which causes the metal burner tip to heat up. The change in flame shape leads to a change in the response factor. The relevant response factor for methane thus rises sharply as soon as the flame becomes wider. When the flame drops to the level of the burner tip, the conversion rate of other hydrocarbons gets better and better, since the metal burner tip gets hotter and hotter. What appears to be a potential advantage quickly turns out to be a disadvantage, since the conversion rate, following a non-linear progression, very quickly becomes so large that this is overcompensated. On passing a threshold value, even small changes lead to a strong response factor change of over 30%. Because strict limits are set for the response factors when approving FIDs for stationary emission measuring devices as well as for exhaust gas measuring systems in the automotive industry, the permissible limit in each case would be exceeded as a result of response factor changes of more than 5 to 10%.
A further problem is that of thermally induced wear of the burner tip and electrode in the combustion chamber. Sensitivity is also lower in the presence of voluminous flames, since ions, which are created in the center of the flame, contribute comparatively less to the measured current flow when the flame gets bigger.